In an optical communications system, optical signals may be generated by semiconductor laser diodes and are transmitted over optical fibers from one location to another. Such optical systems operate most efficiently when the laser diodes and fibers are coupled with minimal power loss.
The basic structure of the optical fiber comprises a core of high purity glass that is surrounded by a cladding glass with a somewhat lower index of refraction. Light launched into the core of the fiber from a laser diode is guided within the core as a result of the index of refraction difference at the core cladding interface.
The laser diode is typically a semiconductor diode laser which includes waveguide structures for laterally and transversely guiding light emitted as a result of applying a pumping current to a thin active layer. Thus, the problem of coupling a laser diode to an optical fiber may be viewed as the problem of coupling two waveguides (i.e., a laser waveguide and an optical fiber waveguide), which waveguides have different optical properties. The dimensions of the laser waveguide are relatively small (e.g., 1 micron), while the fiber core has larger dimensions (e.g., 10 microns). Similarly, the laser has a relatively large numerical aperture (e.g., 0.5) and the fiber has a relatively small numerical aperture (e.g., 0.1). The numerical aperture ("NA") for an optical fiber is approximately equal to the sine of the radiation or acceptance angle of the fiber multiplied by the index of refraction of the material in contact with the entrance or exit face of the fiber. Similarly, for a laser, the numerical aperture is approximately equal to the sine of the radiation angle of the laser multiplied by the index of refraction of the material in contact with the laser emission facet.
The simplest way to couple a laser diode and an optical fiber is to align the fiber in front of the laser diode at close range. However, because of the differences in dimensions and numerical aperture between the laser diode and fiber, such butt coupling is highly inefficient.
The problem of coupling a laser diode and an optical fiber may be solved with improved efficiency by interposing a lens between the fiber and the laser diode. However, in practice, the use of such a lens is difficult. More particularly, alignment of the fiber lens laser combination often requires mechanical adjustments of the fiber to within a fraction of a micron. Such alignment is difficult to achieve and maintain because of the small mass and elongated structure of the fiber.
Another type of coupling mechanism involves use of a spherical reflector. For example, U.S. Pat. Application Ser. No. 053,220, entitled "Fiber Optic Bypass Switch" and filed on May 13, 1987, discloses a system wherein a spherical mirror is used to couple radiation from a first optical fiber to a second optical fiber by reflection. The coupling system of the above identified patent application makes use of a particular imaging property of a spherical mirror. A point source of light, located in the plane at twice the focal length from the spherical mirror and slightly displaced from the center of curvature, is imaged with minimal aberration at a point located in the same plane and symmetrically located with respect to the center of curvature. The imaging plane, in this case, is called the one to one imaging plane. Using this imaging property, light can be imaged from one optical fiber to another when both fibers have end faces located in the one to one imaging plane of the mirror and when both end faces are symmetrically located with respect to the center of curvature of the mirror. Such one to one imaging is optimum for like objects such as two optical fibers. However, the above identified patent application does not disclose a mechanism to optically compensate when coupling two waveguides having disparate optical properties such as an optical fiber waveguide and a laser diode waveguide. A suitable mounting for a directly coupled laser diode and an optical fiber must maintain a separation between the emission facet of the diode and the fiber, thereby to prevent alteration of the desired resonance of the diode. The separation is reduced to a minimum distance, measured in microns, to achieve optimum one to one coupling. Difficulty in maintaining separation is experienced because the mounting will experience thermal expansion when subjected to variable ambient temperature and thermal energy emanating from operation of the diode.
Use of an spherical mirror to couple the diode to a laser is an alternative to direct coupling. However, to couple the diode and the fiber at the imaging plane of a spherical mirror will achieve no increase in coupling efficiency as compared to a directly coupled diode and optical fiber.
In view of the foregoing, it is an object of the present invention to utilize a spherical mirror to couple two unlike waveguides such as an optical fiber and a laser diode in such a way as to compensate for their disparate optical properties.